The type-species of Hyphessobrycon is redescribed and its osteology provided. Hyphessobrycon compressus is distinguished from its congeners by the 41-48 (mode 45) scales in the longitudinal series; 7-9 (mode 9) scales rows between the dorsal-fin origin and the lateral line, and absence of predorsal scales. Additional useful characters include the lack of spots on the body, a dorsal fin with a black spot, and an anal fin with two large, conspicuous hooks, and several other small hooks. Hyphessobrycon milleri is proposed as a junior synonym of H. compressus . The osteology of H. compressus is discussed in detail together with comments about phylogenetic relationships of Hyphessobrycon sensu stricto .

The Neotropical region shows a high degree of freshwater fish diversity (Reis, 2013). Almost all Neotropical fishes are endemic to this biogeographic region, as are several of their families (e.g. , all New World characiform families sensuOliveira et al., 2011, except for Alestidae) and at least one order (Gymnotiformes). Characidae (herein treated sensuMirande, 2010) is the richest family of the Characiformes, but its intra and interspecific relationships remain unclear, with different alternative proposed hypotheses (Mirande, 2009, 2010; Javonillo et al., 2010; Oliveira et al. , 2011). The most problematic genera in the Characidae are Astyanax Baird & Girard, Bryconamericus Eigenmann, Hemigrammus Gill, Hyphessobrycon Durbin, and Moenkhausia Eigenmann, all of which have been repeatedly considered to be polyphyletic.

Hyphessobrycon [from the Greek ύϕήσσων = smaller, and βρυχω = to gnash the teeth, from Brycon Müller & Troschel, a genus of the Characidae (Eigenmann, 1918:172)] was proposed by Durbin (inEigenmann, 1908) as a subgenus of Hemigrammus with a short diagnosis: "this genus is a Hemigrammus with a naked caudal". The type species, by original designation, is Hyphessobrycon compressus (Meek, 1904), from El Hule, Oaxaca, río Papaloapan basin, Mexico, established by Durbin (in Eigenmann, 1908) together with the description of six new species: H. bentosi Durbin, H. compressus milleri Durbin, H. copelandi Durbin, H. melazonatus Durbin, H. panamensis Durbin, and H. serpae Durbin [(= H. eques (Steindachner)]. Eigenmann (1917) subsequently presented a key to the "The American Characidae", including more characters for the recognition of Hyphessobrycon: the lateral line is incompletely pored; the premaxillary teeth are in two series with five teeth in the inner series; the second (= third infraorbital sensuWeitzman, 1962) is separated from the preopercle ventrally; the presence of few maxillary teeth; a naked caudal fin, and the presence of an adipose fin. Eigenmann (1917) also commented, in a pre-cladistic context, that some of the genera diagnosed by these characters are possibly polyphyletic. In agreement with Eigenmannʼs first insight, Hyphessobrycon and other species-rich genera, such as Astyanax , Hemigrammus , and Moenkhausia , are now recognized as polyphyletic (Weitzman & Palmer, 1997; Mirande, 2010; Malabarba et al., 2012). In fact, the species of Hyphessobrycon demonstrate variation in form, color pattern, and size, including some species that do not share all of the characters proposed by Eigenmann (1917) to define the genus. For example, the lateral line is completely pored in some specimens of H. anisitsi (Eigenmann), H. auca Almirón, Casciotta, Bechara & Ruíz Díaz, H. luetkenii (Boulenger), and H. santae (Eigenmann); the adipose fin is absent in H. negodagua Lima & Gerhard, and H. taurocephalus Ellis; and a pseudotympanum is present in H. arianae Uj & Géry and H. elachys Weitzman.

Géry (1977) proposed artificial groups for the species of Hyphessobrycon and considered H. compressus to be a part of "the Hy. compressus group ("true" Hyphessobrycon ) [sic]". This group contains the type species along with H. milleri , suggested by Géry (1977) as "the adult stage of H. compressus ".

Weitzman & Palmer (1997) tentatively proposed the "rosy tetra clade", a group within Hyphessobrycon in which most species have deep, laterally compressed bodies; are usually less than approximately 40 mm SL as adults; have sexually active males with large, high dorsal fins, and elongate anterior anal-fin lobes in many species; and a mostly red or reddish brown body. Carvalho (2011) tested the monophyly of the "rosy tetra clade" within a phylogeny of the Characidae (sensuMirande, 2010) and delimited a monophyletic group that includes most of the species cited by Weitzman & Palmer (1997) including H. compressus .

Hyphessobrycon compressus, as type species of genus, is "the name-bearing type of a nominal genus" (article 67.1 - International Comission on Zoological Nomenclature (ICZN), 1999). Consequently, it is crucial to known in detail the morphology and other characters of this species in order to understand intra generic phylogenetic relationships. We herein provide a detailed description of the osteology, morphometric, and meristic data for H. compressus , a poorly-known species and type species of one of the most species-rich genera of the Neotropical region.

Material and Methods

Measurements and counts follow Fink & Weitzman (1974) and Carvalho et al. (2010). Measurements were made point-to-point with a digital caliper on the left side of the specimen whenever possible, and are presented as percents of standard length (SL) or head length (HL). Counts, in the description, are followed by their frequency in parentheses, including data for type and non-type specimens. An asterisk indicates the count of holotype of Hyphessobrycon compressus . The osteological description is based on 11 cleared and stained (c&s) specimens prepared according to Taylor & Van Dyke (1985). Osteological terminology followed Weitzman (1962), with modifications suggested by previous authors, i.e. , anterior ceratohyal instead of ceratohyal (Zanata & Vari, 2005), posterior ceratohyal instead of epihyal (Zanata & Vari, 2005), autopalatine instead of palatine (Datovo & Vari, 2013), mesethmoid instead of ethmoid (Fink & Fink, 1981), vomer instead of prevomer (Zanata & Vari, 2005), and intercalar instead of opisthotic (Zanata & Vari, 2005). Vertebral counts included the four vertebrae of the Weberian apparatus and the fused PU1+U1 centra of the caudal skeleton as a single element. The patterns of circuli and radii were defined from scales sampled from the region between the lateral line and the insertion of pelvic fin. Catalog numbers are followed by the total number of specimens, number of specimens measured and counted in parentheses, and SL range of all specimens of the lot. In "Material examined" catalog numbers are followed by the total number of specimens, number of specimens measured and counted in parentheses, and SL range of all specimens of the lot (including c&s specimens, if any), when possible. Comparative material examined included also those material listed in Carvalho et al. (2010), Carvalho & Langeani (2013), and Carvalho et al. (2014). Institutional abbreviations followed Reis et al. (2003), with the addition of UFRGS - Departamento de Zoologia, Universidade Federal do Rio Grande do Sul, Porto Alegre. We follow the results of Malabarba et al. (2012: table 1) and Carvalho et al. (2014) for phylogenetic data and discussion according to matrix and characters of Mirande (2010).

Diagnosis. Hyphessobrycon compressus is distinguished from all congeners by the number of scales in the longitudinal series (41-48, mode 45, vs. less than 40 on other species); the number of scales rows between the dorsal-fin origin and the lateral line (7-9, mode 9, vs. 6 or less, rarely 7), and the absence of predorsal scales (vs. scales present). Furthermore, H. compressus can be recognized by the combination of a lack of humeral spots on body, presence of conspicuous bony hooks in anal fin (two large and several small), and hooks absent on pelvic fin.

Description. Morphometric data summarized in Table 1. Small size, maximum of 36.4 mm SL and 46.5 mm total length (AMNH 32275). Body compressed, moderately high, greatest body depth at vertical through dorsal-fin origin. Head profile triangular and rounded anteriorly from lateral view. Dorsal profile of head slightly convex anteriorly, nearly straight from posterior nostril to tip of supraoccipital spine. Dorsal profile of body slightly convex from posterior tip of supraoccipital to base of last dorsal-fin ray, then straight to adipose-fin origin. Ventral profile of body convex from tip of lower jaw to pelvic-fin origin, slightly convex from that point to anal-fin origin, and straight and posterodorsally slanted along of anal-fin base. Dorsal and ventral profile of caudal peduncle straight to slightly concave.

Mouth terminal, premaxillary and dentary approximately same length. Maxilla extending posteroventrally to anterior one-third of orbit almost to contact between second and third infraorbitals, and running approximately at 45 degree angle relative to longitudinal axis of body. Premaxilla short, approximately one-third size of maxilla (Figs. 3b, 4). Nostrils close to each other, anterior opening small and circular, posterior opening twice as large. Nostrils separated by skin flap almost covering posterior nostril. Frontals not united anteriorly, with a triangular fontanel; parietal fontanel large, extending from epiphyseal bar to supraoccipital spine. Eyes moderate in size, without conspicuous adipose eyelid.

Dorsal-fin rays ii,8(3), 9*(141), or 10(1); first unbranched ray approximately 45% of second ray. Dorsal-fin origin at midlength of body or posterior to that point, at vertical through anterior one-third of pelvic-fin base. Base of last dorsal-fin ray at or slightly posterior of vertical through tip of pelvic fin; posterior to anal-fin origin. Tip of longest ray of adpressed dorsal fin extends to adipose-fin origin. Adipose fin slightly semicircular distally, greatest dimension less than orbital diameter; adipose-fin origin between base of fourth and fifth posteriormost branched anal-fin rays; its posterior tip extending to vertical through branched anal-fin end. Pectoral fin with i,10(16), 11*(82), 12(45), or 13(12) rays. Tip of pectoral fin extending beyond to pelvic-fin origin; profile slightly pointed and greatest medially (its base from vertical through opercular bony to horizontal through end portion of third infraorbital). Pelvic fin with i,6(2), 7*(145), or 8(5) rays. Pelvic-fin origin located anterior to vertical through dorsal-fin origin; tip of pelvic fin extending to fourth branched anal-fin ray. Anal-fin rays iii*(71), iv(74), or v(7), 20(3), 21(2), 22*(45), 23(56), 24(35), or 25(13). Anal fin of males with large bony hooks; hooks absent in females. Anal-fin base nearly straight with first unbranched ray sometimes under muscles, but evident in cleared and stained specimens. Last unbranched ray and first two branched largest; remaining rays gradually decreasing in length to sixth branched ray; seventh through last rays approximately of same length. Caudal fin forked, lobes slightly rounded, similar in size, with i,9/8,i*(123) or i,10/8,i(1) rays. Caudal fin naked, other than for scales restricted to base. Basal scales same size as body scales. Dorsal procurrent caudal-fin rays 11(9) or 13(2). Ventral procurrent caudal-fin rays 8(4) or 9(6).

Branchiostegal rays 4(11). First gill arch with 8(1) or 9(1) gill rakers on epibranchial, 1(11) between epibranchial and ceratobranchial, 12(2) on ceratobranchial, and 2(7) or 3(1) on hypobranchial. Precaudal vertebrae 15(6) or 16(4). Caudal vertebrae 17(1), 18(9), or 19(1). Total vertebrae 33(7) or 34(3). Supraneurals 4(1) or 5(9), filiform, other than for some dorsal expansion.

Fig. 5 Hyphessobrycon compressus just after collection, not preserved, left view: (a) nuptial male individual displaying darker color pattern, collected in drainage of río Coatzacoalcos basin, Veracruz Province, Mexico; (b) female from río Usumacinta basin, Chiapas Province, southern Mexico. (c) female from river near Teapa, Tabasco, Mexico. Background of images were edited and differences in fins color are due to back ground color of original images, normally made on hand. (c) Photos (a-b) by Marco Endruweit and (c) by Michael Tobler.

Sexual dimorphism. Mature males can be distinguished from females by the presence of bony hooks on anal fin and by the slightly less concave anterior distal margin of anal fin. Males over 28.0 mm SL have small, slightly developed bony hooks. Males larger than 32.0 mm SL have two pairs of large bony hooks. The first pair is larger, retrorse, fishhook-like and normally located at midlength of last unbranched ray. Length of hook larger than that of ray segment, with posterior tip of hook extending to adjacent ray. Second pair of large bony hooks slightly smaller than first and located immediately before first ramification of branched anal-fin ray. Small bony hooks (size smallest that ray segment) present on unbranched ray and anterior six branched rays of anal fin. Hooks located on distal portion of ray after ray ramification with one pair of bony hooks by ray segment. Tissue developed around large bony hooks of anal fin (Fig. 6). Gill glands (sensuBurns & Weitzman, 1996) on first gill arch in males and females were not macroscopically evident.

Distribution. Hyphessobrycon compressus is distributed in many independent Atlantic slope rivers and streams of southern of Mexico, Belize, and Guatemala, an area known as Middle America (sensuWinker, 2011), in the Usumacinta Province (sensuMiller, 1966, fig. 3) (Fig. 7). It is the most northerly distributed species of Hyphessobrycon . Although there are numerous isolated populations within this range, our analysis revealed they together constitute an unique species without detected variations in meristic and morphometric data among these populations.

Ecological notes. Hyphessobrycon compressus seems to be most frequently found in wetlands, where it is associated with macrophytes, over sandy and muddy substrates (Greenfield & Thomerson, 1997). It is rather sensitive to stress and dies quickly after capture (Endruweit, 2013). In a pool on right side of the road MEX 307, Nuevo Orizaba - Nuevo Reforma, río Usumacinta basin, Mexico, it was abundant in lentic water, with plenty of shade along the margin due to overhanging branches. The water was two meters maximum depth, without aquatic macrophytes, but with terrestrial vegetation hanging into the pool. The bottom was gravel 5 mm, loam and the water highly turbid and greenish (Endruweit, 2013). Mature males have an intense black color to the fins, especially the dorsal and anal fins, but it rapidly loose the coloration after removal of the water, becoming pale as females (Endruweit, 2013) (Fig. 5c). The species is commercialized in the aquarium trade (Alvarado et al., 2005).

Remarks. In the original description of Hemigrammus compressus , Meek (1904) mentioned only the type (F.C.M. 4641, now FMNH 4641), though there is a range of values for some measurement and meristic data in the description, e.g. , "(...) A. 25 to 27; scales 45 to 48; (...) the lateral line incomplete, on the 4 to 10 anterior scales (...) [sic ]". Eigenmann (1918), in the brief redescription of this species, examined eight specimens in three lots: IU 10798 and IU 11126 (now combined in CAS 70114, with five specimens and MCZ 29911, with two specimens), and IU 10929 (now CAS 70116, with one specimen, and with doubts about the locality of collection of this lot and collector), all from Mexico, collected by S. E. Meek. There are, however, no records for these specimens (CAS 70114, CAS 70116, and MCZ 29911) as types.

Géry (1966) presented some data for the holotype of Hemigrammus compressus and mentioned the existence of paratypes, but did not list them (one is MHNG 2181.076; this specimen is cited and jaws illustrated in Géry, 1966: 223, but figure legend is mislabeled as Ceratobranchia binghami ; the same figure is presented in Géry, 1977: 486 and labelled as a paratype of Hyphessobrycon compressus ). In the catalog of FMNH fish types, Henn (1928) did not cite any types to H. compressus . Grey (1947), in turn, listed the holotype and 34 paratypes (23 from El Hule and 11 from Obispo). Ibarra & Stewart (1987) mentioned the holotype and 28 paratypes, from two localities (río Papaloapan and río Obispo). Both, Grey (1947) and Ibarra & Stewart (1987) did not mention the catalog number (with respective specimens number) for these types.

Information added to jar FMNH 105930 by Géry (November 1965) suggested that these specimens could be members of Astyanax . Marilyn Weitzman, in a subsequent analysis, confirmed Géry's information and inserted an undated note identifying the specimens as Astyanax sp. We identify these specimens as Astyanax cf. mexicanus . Vari & Howe (1991) commented that the two specimens in USNM 55728 (probably ex FMNH 4662, now missing - S. Raredon, pers. comm.) are two paratypes of Hemigrammus compressus (26.0-29.5 mm SL). Eschmeyer (2014) listed the holotype and 36 paratypes of Hemigrammus compressus as: BMNH 1905.12.6.4-5 (2); FMNH 4642 (17), FMNH 4662 (11); FMNH 105930 (ex FMNH 4642 - 2); MCZ 29911 (ex IU 11126 - 2), and USNM 55728 (2, missing). All these lots were collected by Meek in March and April 1903. According to ICZN (1999), Articles 72.4.1 and 72.4.1.1., all these specimens may be, in fact, paratypes of Hemigrammus compressus . Furthermore, we add also for type series two lots of California Academy of Science (CAS 70114 and 70116) and one lot at the National Museum of Natural History, Smithsonian Institution (USNM 204387). These were collected by Meek during March-April 1903, as described by Meek (1904), and should therefore be also considered paratypes of H. compressus .

Lima et al. (2003) mentioned the holotype of Hyphessobrycon compressus milleri as "whereabouts unknown". The specimen is housed in the California Academy of Sciences (CAS) and it was with an old identification label of Hemigrammus compressus (Fig. 8). The old collection number in the jar (IU 11255) was metallic and oxidized over time, impregnating the entire specimen with a ferrous color. After careful examination, we found that the holotype of Hyphessobrycon compressus milleri is, indeed, a mature male of H. compressus , with two large bony hooks and black anal fin along its entire length (Fig. 8). The dense chromatophore concentration in this specimen proposed by Durbin, in Eigenmann (1908), as diagnostic for the species, is rather one of the characteristics of mature males of Hyphessobrycon compressus . Furthermore, the jaw length of the holotype of Hyphessobrycon compressus milleri ("maxillary considerably less than the length of the eye" [sic]), is also observed in H. compressus , as is the form of the third infraorbital ("second suborbital covering two-thirds of cheek, otherwise as in H. compressus " [sic]) (compare Figs. 1 and 8). Eigenmann (1918) suggested also that Hyphessobrycon milleri is very close to H. compressus , but deferred in having the dorsal-fin spot extending for entire fin.

Parasphenoid (Figs. 10-11). Unpaired elongate median bone, situated along ventralmost portion of neurocranium. Bordered anteriorly by vomer, posterodorsally by prootic and basioccipital. Parasphenoid with median ventral keel ventrally anterior to area of contact with prootic; posteroventrally it bears two laminar processes that may bifurcate distally. Mid-anterior portion of parasphenoid larger than mid-posterior portion of bone.

Pterosphenoid (Figs. 10-11). Paired laminar bone, trapezoidal from lateral view, with irregular margins, and composing together with orbitosphenoid and rhinosphenoid part of the dorsal portion of ocular cavity. Bordered dorsally by frontal, posteriorly by sphenotic, posteroventrally by prootic and anteriorly by orbitosphenoid. Small foramen of trochlear nerve present at junction of orbitosphenoid with pterosphenoid. Small foramina sometimes present on surface of bone.

Nasal (Fig. 9). Paired tubular bone along anterolateral margin of neurocranium, and parallel to mesethmoid. Bordered anteriorly by premaxilla and posteriorly by frontal. Each nasal carries a portion of laterosensory system (nasal branch), continuation of supraorbital branch in frontal. Anterior and posterior portions of nasal with circular openings.

Asteriscus otolith (Fig. 12). Paired bilaterally symmetrical calcareous concretion and largest otolith in ostariophysans. Round to ovoid with small projections along its border, except for inner margin. Otolith lies in capsule formed by exoccipital and basioccipital. Other otoliths (lapillus and sagitta ) could not be observed and presented because of their thin nature.

Infraorbital 5 (IO5) approximately same height IO4, but anteroposteriorly narrower. Dorsal and ventral margins rounded. IO5 situated over anterior half of hyomandibular, and posteroventrally in contact with sphenotic.

Lower jaw. Dentary (Fig. 3c). Rather robust and long paired bone. Dorsally convex and ventrally almost straight. Bordered posteriorly by anguloarticular and retroarticular; dorsally by maxilla and IO2; posteromedially by coronomeckelian. Laterosensory system with some ventral openings running along ventrolateral surface. Horizontal host bony bridge, bordered anteriorly by dentary, anguloarticular, and Meckelian cartilage at midlength of bone. Small, laminar coronomeckelian near anguloarticular. Single tooth row on dentary. Anteriormost teeth tricuspid, largest, then gradually decreasing posterior to fourth or fifth tooth, then followed by small unicuspid teeth. Central cusp two to three times the size of lateral cusps, and curved inwardly towards mouth. Replacement teeth located behind largest teeth.

Anterior ceratohyal (Fig. 15a). Paired approximately trapezoidal bone, anteriorly narrower; concave along dorsal and ventral margins, with two notches along ventral margin. Bordered anteriorly by dorsal and ventral hypohyal and posteriorly by posterior ceratohyal. First two branchiostegal rays anteromedially connected to small grooves along ventral margin (grooves barely apparent in some specimens); third branchiostegal ray posteriorly near posterior ceratohyal, ventral to medial surface of ceratohyal.

Pharyngobranchials (Fig. 16a). Small bones at anterior limit of each epibranchial. First pharyngobranchial variable in form (triangular, trapezoidal, quadrangular) with cartilaginous caps on anterior and posterior margins. Second pharyngobranchial approximately triangular, with 1-3 very small conical teeth ventrally. Third pharyngobranchial variable in form (triangular, trapezoidal) with teeth along anteroventral half. Fourth pharyngobranchial trapezoidal, cartilaginous, with teeth ventrally, forming the upper pharyngeal plate. Pharyngobranchials gradually increase in size from first to fourth.

Epibranchials (Fig. 16a). Elongated bones of variable form. First three elongate, fourth broad, forming the dorsal portion of each branchial arch. First two epibranchials longest, with uncinate process; third epibranchial with small lateral process in its posterior third; fourth epibranchial with pronounced lateral process, approximately triangular. Gill rakers present on all epibranchials. Epibranchial articulate ventrally with ceratobranchials and medially with pharyngobranchials.

Ceratobranchials (Fig. 16a). Elongate narrow bones of similar form and forming major ventral portion of gill arches. Articulate anteriorly with corresponding hypobranchial and posteriorly with epibranchial (except fifth element). Ceratobranchials gradually decrease in length posteriorly last. First two ceratobranchials with one series of gill rakers and last two with two rows. Dorsolateral row about twice the length of ventrolateral row. Fifth ceratobranchial largest, with gill rakers along anterodorsal margin and with anteroventral half formed by triangular process bearing tiny conical teeth dorsally.

Hypobranchials (Fig. 16a). Small paired, trapezoidal shaped bone with three in series. First hypobranchial with small concavity dorsally and articulated with first basibranchial and first ceratobranchial. Second hypobranchial with more pronounced concavity anterodorsally, and articulated with second basibranchial and second ceratobranchial. Third hypobranchial smaller and narrower anteriorly articulated with third basibranchial and third ceratobranchial. All hypobranchials rakers on ventral surface.

Gill rakers (Fig. 16b). Setiform/conical, with calcified bases on all gill arches. Rakers increase in length from posterior of epibranchial to medial of ceratobranchial and then gradually decreasing thereafter. Small denticles covering ventral half of each raker.

Basibranchials (Fig. 16a). Unpaired, narrow, cylindrical bones with four elements in series situated along median of gill arch. Bordered anteriorly by basihyal and posteromedially by first hypobranchial. Second basibranchial larger than first and bordered anteriorly by first basibranchial and posteriorly by third basibranchial. Elongate third basibranchial, approximately same length as second basibranchial, gradually narrowing posteriorly. Bordered anteriorly by second basibranchial and posteriorly by fourth basibranchial. Fourth basibranchial smallest in series and surrounded by a cartilaginous structure extending to lower pharyngeal plate; bordered anteriorly by third basibranchial and posteriorly by fourth ceratobranchial; cartilaginous plate gradually narrowing posteriorly.

Weber apparatus (Fig. 17). Neural complex approximately triangular with concave posterior margin; base contact third and fourth neural arches. First and second neural arches modified to components of Weberian apparatus. Neural arch 3 short but horizontally elongate and situated below of neural complex with transverse process. Fourth neural arch with posterodorsal concavity, immediately below of neural spine. Tip of first neural spine (corresponding to fourth vertebra), toward posterior portion of neural complex. Tripus triangular, convex along margins and with angular corners. Claustrum small, rather reniform with pointed tips and situated under the scaphium. Scaphium ovoid, posterodorsally inclined, and situated anterior to third neural arch. Intercalarium elongate and relatively pointed anteriorly and posteriorly; dorsally concave ventrally convex, and about three times size of claustrum.

Vertebral column. Precaudal vertebrae (Fig. 18a). Precaudal vertebrae bear neural spine, neural canal, vertebral centrum, and associated with pleural ribs. Fifth to 14th or 15th vertebrae of similar form with one pair of neural prezygapophyses and one pair of neural postzygapophyses, one neural arch, one neural spine, and associated with pleural ribs. Haemal arch with haemal canal of 15th or 16th vertebra associated with small pleural rib. Pleural ribs expanded dorsally, and bordered gradually by lateral surface of each parapophysis then narrowing ventrally.

Caudal vertebrae (Fig. 18b). Each caudal vertebrae formed by neural spine, neural canal, vertebral centrum, haemal canal, and haemal spine. Sixteenth or 17th through 29th or 30th vertebra of similar form with one pair of neural prezygapophyses and one pair of neural postzygapophyses. Last four caudal vertebrae with diminute pre- and postzygapophyses.

Intermuscular bones (Fig. 18). Epineurals between the neural spines range in number between 27(4), 28(1) to 29(4), first through third epineurals not forked, fourth to 24th forked anteriorly, and posterior elements to the hypural plate elongate and flattened. Epipleurals haemal intermuscular bones 17(1), 18(6), or 19(2) with first and second elongate and not forked; third to 10th anteriorly forked, and posterior elements to hypural plate elongate, and flattened.

Extrascapular (Fig. 19). Paired flat small bones bearing branch of laterosensory system along almost half its width, and continuing to supracleithrum. Dorsal portion of extrascapular overlying epioccipital and adjacent to posterior margin of posttemporal. Ventral portion bordered by ventral portion of posttemporal.

Supracleithrum (Fig. 19). Paired flattened to elongated, approximately trapezoidal bone, with dorsal portion concave and slightly rounded. Bordered anteriorly by ventral portion of posttemporal and ventrally by cleithrum and ventral region by postcleithrum 1. Laterosensory system extending through its central portion.

Distal and proximal radials of pectoral fin (Fig. 19a). Paired, flattened to thick bone, relatively trapezoid to irregular forms. Four pairs. Anterior tip of the first to third proximal radials contacting or reaching nearly to scapulum, fourth proximal radial lying near of coracoid. Posterior margin of all proximal radials contacting distal radials. First proximal radial largest, somewhat square, overlying the scapulum with three cylindrical projections posteriorly. Second, third, and fourth proximal radials circular/ovoid and approximately of equals length and shape. First through third distal radials of same size. Fourth distal smaller, about half the diameter of others. Distal radials contacting with the pectoral-fin rays.

Anal fin (Fig. 6). Anal fin formed by proximal, medial, and distal radials plus unbranched and branched rays. First four branched anal-fin rays lack medial radials and first three unbranched lack distal radials. Proximal radial elongate with anterior expansion of first proximal radial. First to fourth proximal radials situated between haemal spines of first and second caudal vertebrae, or first proximal radial between haemal spines of last precaudal vertebrae and second to fourth between first and second caudal vertebrae. Proximal radials bordered dorsally by medial radials. Medial radials approximately square, dorsally contacting proximal radials and ventrally distal radials. Distal radials ovoid, some dorsally pointed projections and bordered ventrally by anal-fin rays. First unbranched rays small; rays increasing in length to ultimate unbranched ray. Last unbranched ray and first two branched rays largest, other branched rays decreasing gradually in length to last branched ray. Small- to large-sized bony hooks on anal-fin rays. Two pairs of large bony hooks: one on last unbranched ray and other in first branched ray. Hooks on median portion of anterior fin ray. Large hook on last unbranched ray retrorse, largest than the size of its segment. Posterior tip of hook extending to subsequent ray. Hook on first branched ray slightly slender than anterior hook, but larger than size of ray segment, and also extending to subsequent ray. Large hooks positioned midlength of ray, proximate to branched portion of rays. Small-sized hooks, smaller than the size of associated segment, on second to sixth branched ray along both anterior and posterior branches.

Caudal fin and hypural complex (Fig. 22). Caudal fin consists of dorsal and ventral procurrent rays, epurals, uroneural, hypurals, unbranched and branched caudal-fin rays, and parhypural. Procurrent rays gradually increasing in length posteriorly with the first seven rays associated with neural spines of 32nd or 33rd vertebrae, three associated with two epurals, and the last associated with urostyle. Dorsal unbranched ray of caudal fin associated with sixth hypural; two branched rays with fifth hypural, two branched rays with fourth hypural, five rays with third hypural, two with second hypural; five with first hypural; one with parhypural. Ventral unbranched ray and last ventral procurrent ray associated with 32nd or 33th vertebrae, and remaining ventral procurrent rays with 31st or 32nd vertebrae. Epurals elongate, relatively cylindrical. Uroneural with anteroventrally convex neural process, with irregular anterior surface. Hypurals 1 and 3 approximately triangular, flattened. Hypurals 2 and 4, 5, and 6 approximately trapezoidal. Hypurals 2 and 4 of approximately same size, others gradually decreasing in length. Hypural 2 connected with last vertebra. Hypural 3 with anterior tip at base of uroneural, and other hypurals with free anterior margins, contacting or not uroneural (hypurals 4, 5, and 6) and parhypural (hypural 1). Parhypural with small haemal process.

The type species of Hyphessobrycon was hypothesized by Weitzman & Palmer (1997) to belong to a group of species of the "rosy tetra clade". This group was found to be monophyletic by Mirande (2010, clade 292), but specimens of H. compressus were not available to Mirande (2010) and this species was not included in his analysis. Malabarba et al. (2012) and Carvalho et al. (2014) performed a new analysis including H. compressus and found it belongs to clade 292 of Mirande (2010), which also included Hemigrammus unilineatus (Gill), Hyphessobrycon eques , H. pulchripinnis Ahl, H. socolofi , Weitzman, and Pristella maxillaris (Ulrey). In the first of the phylogenetic analyses, Hemigrammus unilineatus , the type species of that genus, was recovered within the same clade of Hyphessobrycon compressus (Malabarba et al. , 2012; Carvalho et al. , 2014), but in an ongoing more inclusive phylogenetic analysis Hemigrammus unilineatus resolves in a separate clade and does not seems to be part of Hyphessobrycon sensu stricto . Autapomorphies for H. compressus , according to Malabarba et al. (2012) are: the anterior paired projections of the parasphenoid are absent; a lack of pelvic-fin bony hooks in adult males; and the predorsal scales leaving a naked area anterior to the dorsal fin. Unlike other Hyphessobrycon species, H. compressus has small scales, 41-48 in a longitudinal series, and 7-9 scales above lateral line, the highest values for a species of Hyphessobrycon . Other Hyphessobrycon species normally do have no more than 40 scales in the longitudinal series that includes the lateral lines, and the scales rows between dorsal-fin origin and lateral line usually do not exceed six.

The median predorsal scales series is composed of regularly or irregularly arranged scales in a row between the supraoccipital process and the dorsal-fin origin. This is the common condition in Characidae incertae sedis (sensuLima et al., 2003). Gymnocorymbus Eigenmann, however, is diagnosed by the absence of a predorsal scale series. Benine (2004) also mentioned other species in the Characidae with the same condition, e.g. , Jupiaba apenima Zanata, J. pirana Zanata, J. poranga Zanata, J. yarina Zanata, Moenkhausia levidorsa Benine, besides some specimens of Astyanax aff. bimaculatus . Hyphessobrycon compressus is, however, the unique species of the genus lacking median predorsal scales.

Color patterns have been suggested convey a phylogenetic signal among recently described species of Hyphessobrycon (e.g. , Bertaco et al., 2007; Benine & Lopes, 2008; Zanata & Camelier, 2010; Ingenito et al., 2013; Dagosta et al., 2014; Lima et al., 2014). One pattern was proposed as a putative synapomorphy for part of Hyphessobrycon (the "rosy tetra clade", Weitzman & Palmer, 1997) including H. compressus . Most Hyphessobrycon species have humeral spots in the flank, but such pigmentation is missing in H. compressus and part of the "rosy tetra clade" of Weitzman & Palmer (1997). Besides of this character present in some species, the black dorsal fin and no midlateral stripe on body, may be useful in the identification of Hyphessobrycon sensu stricto , including the type species H. compressus , H. bentosi , H. copelandi , H. epicharis Weitzman & Palmer, H. eques , H. erythrostigma (Fowler), H. georgettae Géry, H. haraldschultzi Travassos, H. hasemani Fowler, H. khardinae Zarske, H. megalopterus (Eigenmann), H. micropterus (Eigenmann), H. minor , H. pulchripinnis , H. pyrrhonotus Burgess, H. rosaceus , H. roseus (Géry), H. simulatus (Géry), H. socolofi , H. sweglesi (Géry), H. takasei Géry, and H. werneri (Géry & Uj), besides 'Cheirodon ' troemneri Fowler (Carvalho, 2011). Other species recently referred to the "rosy tetra clade", as H. dorsalis Zarske, H. jackrobertsi Zarske, H. paepkei Zarske, and H. pando Hein, share these characteristics, but their taxonomic status are doubtful. The more than 100 remaining species of Hyphessobrycon other than those in the rosy tetra clade probably will need to be assigned to other genera or new genera (Hyphessobrycon sensu lato ). An example of this rearrangement is Ectrepopterus uruguayensis (Fowler), formerly housed in Hyphessobrycon , but that failed to group with H. compressus and wasfound more related to other characid taxa in a more representative and robust analysis (Malabarba et al., 2012). We may expect the same for other species currently listed in Hyphessobrycon .

The small characids, in special the incertae sedis (sensu Lima et al., 2003), have a conservative morphology associated to a long period of evolution of the group (Weiss et al., 2012). Morphological changes in different lineages seems to be convergent resulting in high levels of morphological homoplasies, that may be associated with small size and reduction or loss of structures, turning difficult to find robust monophyletic clades (Mirande, 2010; Mattox et al., 2013). Hyphessobrycon compressus has a skeleton similar to those described for other members of Characidae [Astyanax fasciatus (Cuvier) by Mejía-Mójica & Díaz-Pardo (1991); A. mexicanus De Filippi (type species) by Valdéz-Moreno & Contreras-Balderas (2003); Bramocharax caballeroi Contreras-Balderas & Rivera-Teillery by Valdez-Moreno & Contreras-Balderas (2009); Brycon meeki Eigenmann & Hildebrand by Weitzman (1962); Bryconamericus exodon Eigenmann (type species) by Serra & Langeani (2006); Carlastyanax aurocaudatus (Eigenmann) (type species) by Ruiz-C. & Román-Valencia (2006); Cyanogaster noctivaga Mattox, Britz, Toledo-Piza & Marinho (type species) by Mattox et al. (2013); Gymnocharacinus bergi Steindachner (type species) by Miquelarena & Arámburu (1983); Gymnocorymbus ternetzi (Boulenger) by Bogutskaya et al. (2008)]. Osteological differences among these species have been briefly discussed or debated (e.g., as table 2 of Valdez-Moreno & Contreras-Balderas, 2009), and is potentially useful in a future more comprehensive phylogenetic analysis. Overall, most differences are associated with teeth (form, number, and position), shape and position of bones, and presence vs. absence of structures. Hyphessobrycon compressus has unique and distinctive characters among its congeners, and the understanding of synapomorphies shared with other species may provide examples of these and allow the recognition of the Hyphessobrycon sensu stricto .

Four Hyphessobrycon species are recognized valid in the Middle America and southern Mexico: H. compressus (including H. milleri as its junior synonym proposed herein), H. panamensis Durbin, H. tortuguerae Böhlke, and H. savagei Bussing. Hyphessobrycon compressus readily differs from the others in the possession of a black dorsal fin (vs. hyaline dorsal fin). Freshwater fish diversity is uneven across Central America. Miller (1966) proposed that the dispersion of the Central American "primary" freshwater fishes (Characiformes, principally) into the region occurred during the Plio-Pleistocene emergence of the Panamanian land bridge, whereas "secondary" freshwater fishes (Poeciliidae and Cichlidae, especially) dispersed and diversified in Central America during the early Cenozoic, long before the Plio-Pleistocene rise of the Isthmus of Panama (Matamoros et al., 2015). The Characidae is most diversified in lower Central America (Matamoros et al. , 2015), but H. compressus occurs in the upper portion of Middle America and southern Mexico, in the Grijalva-Usumacinta, Polochic-Cahabon, and Motagua-Nombre de Dios areas of endemism (sensuMatamoros et al. , 2015). Almost all species of Hyphessobrycon of the ‟rosy tetra cladeˮ are in South America, and it is probable that ancients of H. compressus had a fragmentation of a widespread ancestral species or dispersed from northern South America into middle Central America, as noted for others Characiformes [Cyphocharax Fowler and Lebiasina (Valenciennes)] (Matamoros et al. , 2015).